Hey peeps, well under way with a top secret map project

Just curious what the best option for dealing with the hundreds of hidden faces, they will all be a a single texture that I will replace when im finished. Does 'No Draw' remove then from the loop completely? Also hollow unused spaces, will they be more efficient if filled out?

Thanks for reading Any tips or further comments on map efficiency would be gratefully recieved, Im planning to max out out on physics props.